1. Technical Field of the Invention
The present invention relates in general to the mobile radio communications field and, in particular, to a method for estimating the speed of a mobile station in a cellular communications system.
2. Description of Related Art
In order to increase the capacity of cellular communications systems, relatively small cells referred to as "microcells" are being used. In order to handle much higher traffic requirements, the size of these cells has been decreased to a significant extent. Unfortunately, as the cell sizes are decreased, mobile stations operating in these systems cross the cell borders more frequently, which results in an increase in the number of handovers between cells. This handover problem is compounded by faster moving mobile stations, which cause an exceedingly high number of handovers as the cell sizes become very small.
An approach to solving this handover problem is to overlay the smaller cells with larger cells in a hierarchical manner. Consequently, the faster moving mobile stations can be assigned to a layer having larger cells, and the slower moving mobile stations can be assigned to a small cell layer.
In this regard, if the speed of the mobile stations could be accurately estimated, then the faster and slower moving mobile stations could be readily distinguished and placed in the appropriate cell layers. Notably, such a hierarchical cell structure would increase the overall system capacity significantly, all the while keeping the handover rate at a reasonable level.
Nevertheless, a problem with using such a layered cell approach with existing digital cellular communications systems is that there has been no method for accurately estimating the speed of a mobile station. For example, the European Global System for Mobile Communications (GSM) is a digital cellular communications system, which uses Time Division Multiple Access (TDMA) techniques to divide the carrier signals into time slots. The information transmitted or received during these time slots occurs in "bursts". For added diversity and secure communications, the GSM carrier bursts are frequency hopped. Consequently, there is no continuous signal available to be received in a GSM system. Therefore, the conventional methods that utilize a constant carrier envelope to estimate a mobile station's speed (e.g., "level crossing rate" method) cannot be used for systems like the GSM. Since the GSM and many other cellular systems use TDMA combined with frequency hopping, any technique used to estimate the speed of a mobile station in such a system would have to be able to utilize the information received during one burst, in order to provide an accurate estimation.
In typical urban environments (e.g., cities) where relatively high system capacities are needed, much smaller cells are being used. However, as mentioned earlier, the use of such smaller cells has produced a significant increase in handovers. Therefore, in order to alleviate the small cell/handover problem, a method is needed for accurately estimating the speed of a mobile station in a GSM or similar type of cellular system.